a) Field of the invention:
The present invention relates to a zoom lens system, and more specifically to a zoom lens system which is to be used with cameras incorporating electronic image pickup devices.
b) Description of the prior art:
Under the recent circumstances where compact and light weight cameras are manufactured at low costs for domestic use, it is desired to develop compact and light weight photographic lens systems which can be manufactured at low costs.
As a compact zoom lens system having a simple composition, there is known the lens system disclosed by Japanese Patent Kokai Publication No. Sho 56-123512.
The conventional zoom lens system of this type comprises, in the order from the object side, a first lens unit having a negative refractive power as a whole and a second lens unit having a positive refractive power as a whole, and is adapted so as to perform zooming by moving the first lens unit and the second lens unit along the optical axis while changing the total length of the lens system. Since a remarkable change of the total length produces undesirable effects to complicate the lens barrel to be adopted for the lens system and impose restrictions on designs of cameras to be used with the lens system, magnification of the second lens unit of the above-mentioned zoom lens system is determined so as to minimize the change of the total length of the zoom lens system.
As a zoom lens system which has a total length unchanged for zooming and a simple composition, there is known the zoom lens system proposed by Japanese Patent Kokai Publication No. Sho 63-292106. The zoom lens system of this type comprises a first lens unit which is kept stationary for zooming and has a negative refractive power as a whole, a second lens unit having a positive refractive power as a whole and a third lens unit having a positive refractive power as a whole, and is adapted so as to perform zooming by moving the second lens unit and the third lens unit along the optical axis.
The zoom lens system preferred as an embodiment of the zoom lens system proposed by Japanese Patent Kokai Publication No. Sho 63-292106 has a long total length L. Speaking more concretely, when the focal length of this zoom lens system at the wide position and the tele position thereof are represented by f.sub.W and f.sub.T respectively, the zoom lens system has ##EQU1## on the order of 6.7 to 10. In order to design this zoom lens system more compact, it is therefore necessary to shorten the total length of the lens system. When the refractive power of each lens element of the zoom lens system is strengthened for shortening the total length thereof, however, aberrations will be aggravated undesirably.